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Posted: July 17, 2006

Self-illuminating quantum dots for in vivo imaging

(Nanowerk News) Quantum dots have already proven themselves as powerful imaging probes that can enable researchers to track disease-related molecules inside cells and even within the body. In response to illumination with near infrared light, these semiconductor nanocrystals shine so brightly it is possible to spot individual quantum dots inside cells.

Now, however, investigators at the Center for Cancer Nanotechnology Excellence (CCNE) Focused on Therapy Response, based at Stanford University, have developed a self-illuminating quantum dot that can reveal its presence without an external light source. This team was able to create this novel quantum dot using a new method they developed for adding proteins to the quantum dot surface. This work was reported in the journal Angewandte Chemie International Edition ("HaloTag Protein-Mediated Site-Specific Conjugation of Bioluminescent Proteins to Quantum Dots").

Jianghong Rao, Ph.D., a member of the Stanford CCNE, and Sanjiv Gambhir, M.D., Ph.D., principal investigator there, led the team that developed a generalized method for creating a uniform protein coating on the surface of quantum dots. This method uses a commercially available, genetically engineered enzyme known as a HaloTag protein, that forms a strong chemical bond between itself and a small organic molecule – the HaloTag ligand – that is easily coated onto a quantum dot surface. Furthermore, the HaloTag protein itself can be linked to other proteins, providing an easy method for adding such proteins to a quantum dot surface.

To demonstrate the power of this method, the investigators first attached the bioluminescent protein luciferase to the HaloTag protein. They then mixed this protein construct with a quantum dot coated with HaloTag ligand to form the self-illuminating quantum dot. Adding luciferase’s substrate, a molecule known as coelenterazine, to the protein-coated quantum dot produced two wavelengths of bright fluorescent light – one wavelength produced by luciferase, the other by the quantum dot.

The researchers note that this technique can be used to add a wide variety of proteins, including tumor-targeting ligands, to the surface of quantum dots. They also comment that the tagging reaction occurs under mild physiological conditions, suggesting that this process could be used to chemically label cells, too, without damaging their viability.